Method and apparatus for water recovery from saline solutions
Patent Information
- Application Number
- PCT/US2026/020605
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-03-25
- Filing Date
- 2026-03-24
- Publication Date
- 2026-10-01
Smart Images
Figure US2026020605_01102026_PF_FP_ABST
Abstract
Description
Attorney Docket No. 0036709-000300 METHOD AND APPARATUS FOR WATER RECOVERY FROM SALINE SOLUTIONSCROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of United States Provisional Patent Application No.63 / 777,411, filed on March 25, 2025. That application is incorporated by reference herein.STATEMENT OF GOVERNMENT SUPPORT
[0002] Not applicable.FIELD OF THE INVENTION
[0003] Embodiments reported herein generally relate to water recovery from aqueous solutions by evaporation.BACKGROUND
[0004] Water is recovered from many solutions, across numerous industries, using a variety of technologies. Historically steam driven evaporators have been implemented for over a century, with some of the first practical applications being the concentration and production of commodity products such as sugar and salt. The applications for steam-driven evaporation technology expanded throughout the industrial age into new markets such as chemical, petroleum, pulp & paper, food & beverage, pharmaceuticals, desalination, among others. In such evaporation processes, steam was added from an external source, such as a boiler, as a heating medium to evaporate the water fraction of the process stream.
[0005] One of the earliest enhancements of evaporation technology was driven by the desire to reduce the external steam demand. This enhancement was called multiple-effect evaporation.Multiple effect evaporation has multiple stages of evaporators arranged such that the vapor generated in the first evaporator effect will be used as the heating steam in the second evaporator effect. Vapor generated in the second effect is used as heating steam for the third effect and so on. Vapor generated in the last effect is condensed in a vapor condenser with cooling water used as the heat sink. In such multiple effect arrangements, the steam demand required for evaporation is generally described by equation 1 with a graphical representation in FIG. 1. In this way, adding a second evaporation effect reduces the external heat demand by 50%. Adding a third evaporation effect reduces the external steam demand by 67%.(Mass of Evaporation) n ~ (Mass of Heating Steam) ( 1 ) in which ‘n’ is the number of evaporator effects used in the design. In such configurations, it is known that there would be a natural optimal limitation to the number of evaporator effects in whichAttorney Docket No. 0036709-000300 the additional capital expenditure needed for additional evaporator effects would not offset the operational cost savings associated with the reduction in steam demand. In real-life application, the reduction of steam consumption will have a definite optimized endpoint given the need to balance operational costs with capital costs.
[0006] With growing public concern on the carbon footprint associated with utilizing fossil fuels, it has become a growing trend to avoid fossil fuel driven operations and instead favor designs that are driven primarily with electrical power generated from renewable energy resources such as solar, hydro, or the like. In the realm of industrial evaporation, the use of electrical power to drive evaporation is commonly achieved via two methods: electrical heating elements and mechanical vapor compression.
[0007] Heating elements can be used as the energy source for boilers in lieu of natural gas or other fossil fuels. These are called electric boilers. However, electric boilers are frequently impractical to provide steam for continuous operation of an evaporator. The steam requirements are usually high enough that local electrical infrastructure cannot accommodate such a large electric power load. Electric boilers consume between 570 - 740 kW / metric ton of evaporation. This method becomes impractical for the majority of applications in which the evaporation duty is significant.
[0008] Alternatively, mechanical vapor compression (MVC) is a highly efficient process using mechanical energy input to achieve evaporation and condensation consuming between 12 - 25 kW / metric ton of evaporation. The fundamental difference between the vapor compression design and the conventional steam driven evaporator design is that the latent heat invested into the evaporated vapor is fully recovered in the evaporator. Since the evaporator functionally performs both evaporation and condensation, essentially all the latent heat is recycled, with no loss of heat to cooling water or other heat sink. This leads to the operating costs for evaporators operating via MVC technology to be much lower than evaporators operating on steam generated through electric boilers or even fossil fuels.
[0009] Depending on the operating conditions of the plant, a quantity of additional steam is commonly required to maintain the overall evaporator heat balance and to ensure stable operating conditions. This quantity of additional (makeup) steam in a conventional mechanical vapor compression driven system is influenced by several process conditions. One of the key drivers for the need for external heat is the concentration factor of the evaporation system. The concentration factor (CF) is described by the following equation:(Mass of Feed)z(Mass of Concentrate) = CF (2)Attorney Docket No. 0036709-000300
[0010] See FIG. 2 for an example of how concentration factor effects the need for makeup steam; some evaporation system designs operate in this window of low CF in which it is practically not possible to satisfy the heat balance without adding external heat with this conventional design. Due to most of the energy in an MVC system being the latent heat of vaporization recycled, when only a small fraction of the water in the process stream is evaporated and compressed in the MVC there is not enough energy put into the system when raising the vapor pressure to preheat the large volume of feed to operating temperatures see FIG. 3 for the BFD of an example energy balance of this scenario in Table 1 below.Table 1: Heat Balance of a Conventional MVC Driven Evaporator Without Heat Recovery Heat Balance of a Conventional MVC Driven Evaporator Without Heat RecoveryFlow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) (’F) (mmbtu / hr) Influent HeatFeed 1,800,000 20.00% 0 165 197,533,727 MVC Power - — 3,679 — 9,363,247 Circulation Pump Power -- — 220 — 560.857 Total - — — — 207,457,830 Effluent HeatConcentrated Solution 1,607,696 22.39% — 227.40 252.634.914 Vent Vapors 1,913 — — 237.42 2,218,371 Condensate (Recovered Water) 190.391 0.00 — 237.42 39,110,201 Heat Loss -- — — — 1,903,914 Total - — — — 295,867,400 Heat Balance — — — — -88,409,570Makeup Steam Required 76,240.3 — — 237.42 88,409,570
[0011] Improvements to the consumption of make-up steam required can be achieved by adding heat exchangers that are designed to recover heat from the process streams being discharged from the evaporator system. However, limitations in heat exchanger design and reliability for recovering heat from the effluent streams can ultimately lead to a reduction in overall energy deficit without completely eliminating the problem.
[0012] This heat recovery limitation can be understood by considering the approach temperature that is practically possible for the heat recovery heat exchangers. The approach temperature is the difference in temperature between the influent aqueous solution (cold side) and the effluent distillate and effluent concentrate streams (hot side). Approach temperatures for these heat recovery heat exchangers are typically in the range of 10 °F to 30 °F, with smaller approach temperatures beingAttorney Docket No. 0036709-000300 favorable to the heat balance. Tighter approach temperatures would deliver improved heat recovery but often that is not practical due to the significant impact on heat exchanger size and capital cost.
[0013] For applications where only a small vapor fraction is being extracted from the process stream, this approach temperature limitation causes a significant deficit in heat balance in the system. The quantity of energy leaving with the concentrate and distillate streams is then balanced by adding external heat to maintain steady-state operation of the evaporation system. See FIG. 4 for a BFD of the example heat balance of a low recovery system with a 10 °F approach as described in table 2.Table 2: Heat Balance of a Conventional MVC Driven Evaporator with Heat RecoveryHeat Balance of a Conventional MVC Driven Evaporator With Heat RecoveryFlow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) fF) (mmbtu / hr) Influent HeatFeed 1,800,000 20.00% 0 165 197,533,727 MVC Power - — 3,260 — 8,297,087 Circulation Pump Power -- — 220 — 560.857 Total - — — — 206,391,670 Effluent HeatConcentrated Solution 1,607,696 22.39% — 175.00 184.886.355 Vent Vapors 1,913 — — 234.60 2,216,465 Condensate (Recovered Water) 190.391 0.00% — 175.00 27,225,970 Heat Loss -- — — —Total - — — — 216,232,704 Heat Balance — — — — -9,841,034Makeup Steam Required 8,493.7 — — 234.60 9,841,034
[0014] To design for a closer approach temperature becomes impractical as the surface area requirements of the preheaters are excessively high leading to high capital costs associated with the heat recovery heat exchangers. See FIG. 5 for a BFD of an example of how close these approach temperatures need to be to balance the energy of the system as described in Table 3.Table 3: Heat Balance of a Conventional MVC Driven Evaporator with Impractical Heat Recovery Heat Balance of a Conventional MVC Driven Evaporator With Impractical Heat Recovery Flow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) (°F) (mmbtu / hr) Influent HeatFeed 1,800,000 20.00% 0 165 197,533,727MVC Power - — 2,876 — 7,319,090Attorney Docket No. 0036709-000300 Circulation Pump Power -- — 220 — 560,857 Total - — — — 205,413,673 Effluent HeatConcentrated Solution 1,607,696 22.39% — 167.70 175,448,101 Vent Vapors 1,913 — — 232.00 2,227,108 Condensate (Recovered Water) 190.391 0.00% — 167.70 25,834,658 Heat Loss -- — — — 1,903,807 Total - — — — 205,413,673 Heat Balance - — — — 0Makeup Steam Required 0.0 — — 232.00 0
[0015] In many applications, the feed stream also contains a constituent that scales or fouls the heat exchanger which leads to a loss of performance, a higher operating approach temperature which results in an energy deficit in the system requiring make-up steam to compensate. These heat exchangers would need to be taken off-line for cleaning to restore their performance, but this has the drawbacks of operational downtime and expense. This commonly applies to both the concentrated aqueous stream leaving the system as well as the aqueous feed stream entering the system.
[0016] For example, in evaporator systems processing saline solutions the concentrate leaving the evaporator is very close to saturation and recovering energy from cooling the concentrated brine would lead to precipitation of mineral salts in the heat exchanger which leads to plugging, meaning significant heat recovery on the concentrated stream is not possible. This is especially true for normally soluble salts such as sodium chloride and potassium chloride among others. An example heat balance is presented in Table 4 with an associated BFD depicted in FIG. 6 to show the effect of not recovering heat from the concentrate on the system.Table 4: Heat Balance of a Conventional MVC Driven Evaporator with Partial Heat Recovery Heat Balance of a Conventional MVC Driven Evaporator With Partial Heat RecoveryFlow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) CF) (mmbtu / hr) Influent HeatFeed 1,800,000 20.00% 0 165 197,533,727 MVC Power — — 3,616 — 9,203,931 Circulation Pump Power — — 220 — 560,857 Total - — — — 207,298,515 Effluent HeatConcentrated Solution 1,607,696 22.39% — 227.40 252,634,914 Vent Vapors 1,913 — — 237.00 2.218.088 Condensate (Recovered Water) 190,391 0.00% — 175.00 27,225,970Heat Loss - — — — 1,903,914Attorney Docket No. 0036709-000300 Total -- — — — 283,982,886 Heat Balance - — — — -76,684,371 Makeup Steam Required 66,137.5 — — 237.00 76,684,371
[0017] In another example, consider a case in which the aqueous feed solution contains quantities of slightly soluble compounds such as calcium carbonate, calcium sulfate among others, that possess a solubility that is inversely proportional to temperature. That is, their solubility decreases with an increase in solution temperature. In conventional evaporators this becomes problematic as feed preheaters as well as heat transfer surfaces in the evaporator itself will be subject to scaling with these inversely soluble compounds. Historically, engineers have attempted to remedy this issue through several different methods. The most straightforward approach is to remove the scaling components upstream through pretreatment. This typically has high operating costs associated with high consumption of chemicals required for the precipitation reaction as well as for the disposal of the solid-phase sludge that is generated as a waste stream from this process.
[0018] In cases where the costs associated with removal of scaling species is prohibitive, it then becomes necessary to configure the system for allowances that compensate for the operational downtime caused by scaling. This can take the form of large tankage volumes upstream and downstream of the evaporator to act as a buffer when the evaporator must be taken off-line for removal of the scale that has formed. Scale removal can be done chemically or mechanically and can result in off-line time on the order of several days or more. In some cases, where the application is intolerant to the loss of evaporator performance, a second evaporator train can be installed in parallel such that one evaporator remains on-line when the second evaporator is in an offline cleaning. Both allowances, whether large tank volumes or standby evaporator trains, can lead to significant levels of additional capital cost.BRIEF SUMMARY OF THE INVENTION
[0019] Embodiments of the invention may alleviate the issues of significant demand of external makeup steam as well as the issue of temperature-based scale formation by maintaining the temperature of the process solution at a constant temperature, without significant heat or cooling, which mitigates the scale potential of the scaling compounds and can be used to balance the heat of the system.
[0020] The technology reported herein may overcome the limitations of conventional methods of recovering water from a saline solution that are capable only through the use of makeup steam and costly feed preheaters. For example, one embodiment provides a method that includes (a)Attorney Docket No. 0036709-000300 providing a water stream as feed to an evaporator where the water stream consists of soluble inorganic and / or organic compounds; (b) feeding the water stream to an evaporator; (c) drawing off a fraction of the water stream as water vapor; (d) compressing the water vapor and directing the compressed water vapor to the evaporator tubes to as the energy source for evaporation; (e) controlling the evaporator pressure such that the compressed water vapor condenses on the evaporator tubes and maintains a positive heat balance; (f) removing the condensed water vapor out of the evaporator as liquid water; and (g) removing the effluent water stream from the evaporator at a higher concentration relative to the feed stream.
[0021] Tn some embodiments of the invention, the concentration factor of the evaporator is less than 1.5, less than 2.0, less than 2.5, less than 3.0, less than 3.5, less than 4.0, less than 4.5, less than 5.0, less than 5.5, or less than 6.0. In some embodiments the concentration factor of the evaporator is between 1.75 and 2.25. In some embodiments the concentration factor of the evaporator is between 4.75 and 5.25.
[0022] In some embodiments the evaporator is a vertical falling film evaporator. In other embodiments it is a horizontal falling film evaporator, forced circulation evaporator, flash evaporator, rising film evaporator, or draft tube evaporator. The evaporator may be a multi-effect evaporator system.
[0023] In some embodiments the water stream is fed to the top head of the evaporator, travels down the tubes and exits from the bottom head, such that the brine moves through the evaporator one time without recirculation.
[0024] In some embodiments brine is partially circulated from the evaporator bottom head back to the top head of the evaporator. In some embodiments, “partial” circulation is circulation of about 5% to 20% of the brine.
[0025] In some embodiments, the recirculating brine of the evaporator, is used as the heat sink for condensing the water vapor in an external condenser.
[0026] In some embodiments the vent is removed from the evaporator process in favor of a vacuum system.
[0027] In some embodiments heat recovery heat exchangers are used on the effluent streams, allowing for a higher operating pressure to achieve heat neutrality of the system.BRIEF DESCRIPTION OF THE DRAWINGSAttorney Docket No. 0036709-000300
[0028] FIG. 1 shows relative steam demand of a multiple effect steam driven evaporator system as function of the number of effects.
[0029] FIG. 2 through FIG. 6 are example energy balances showing a comparison of conventional evaporation methods paired with differing heat recovery devices to the invented method for the same process duty. These examples paint a picture of the limitations faced by conventional methods of evaporation and heat recovery for a system with a low concentration factor and how the invented method overcomes them.
[0030] FIG. 2 shows impact of concentration factor on the evaporator heat balance. This graph shows the makeup steam requirements for a process as a function of the concentration factor. The basis of the trend is as follows. The evaporation rate is held constant at 30,000 Ib / hr. The effluent concentration and fluid properties are held constant. The feed temperature is held constant at 77 °F. There is heat recovery on the distillate stream with a 10 °F approach. It is assumed heat recovery cannot be performed on the blowdown stream, as is often the case in industry. The feed concentration and rate are altered to adjust the CF.
[0031] FIG. 3 shows recovery of water from a saline solution using mechanical vapor compression in a conventional vertical tube falling film evaporator without heat recovery. In this method make up steam is required to close the heat balance of the system.
[0032] FIG. 4 shows recovery of water from a saline solution using mechanical vapor compression in a conventional vertical tube falling film evaporator. In this method preheaters are used on the feed and blowdown stream to recover heat using a 10 °F approach but make up steam is still required to close the heat balance of the system
[0033] FIG. 5 shows recovery of water from a saline solution using mechanical vapor compression in a conventional vertical tube falling film evaporator. In this method preheaters are used on the feed and blowdown stream to recover heat using a 2.7 °F approach temperature. This system is energy positive and does not require makeup steam.
[0034] FIG. 6 shows recovery of water from a saline solution using mechanical vapor compression in a conventional vertical tube falling film evaporator. In this method feed preheaters are used on condensate only due to precipitation risk upon cooling the condensate. Makeup steam is required to close the heat balance of the system.Attorney Docket No. 0036709-000300
[0035] FIG. 7 shows recovery of water from a saline solution using an embodiment that includes using mechanical vapor compression in a vertical tube falling film evaporator with a vacuum system on the shell side vent to control the effluent temperature to the same value as the feed. In this method no makeup steam or feed preheaters are required to close the heat balance of the system.
[0036] FIG. 8 shows a process flow diagram (PFD) of an embodiment of the invention.
[0037] FIG. 9 shows a process flow diagram (PFD) an embodiment of the invention.
[0038] FIG. 10 shows a process flow diagram (PFD) an embodiment of the invention.DETAILED DESCRIPTION
[0039] As used herein, “falling film evaporator” is a type of evaporator that has vertical tubes with the process fluid being fed to the top of the evaporator where it can then fall as thin film on the inside of the tubes. Heat is transferred via condensing water vapor to the process fluid on the shell side of the evaporator.
[0040] As used herein “horizontal tube falling film evaporator” is a type of evaporator that has horizontal tubes. With process feed being fed to the top of the evaporator where it can then fall as a thin film on the outside of the tubes. Heat is transferred via condensing water vapor to the process fluid on the tube side of the evaporator.
[0041] As used herein, “circulation pump” means a pump used in evaporator systems to circulate process fluid from a sump to the top of an evaporator.
[0042] As used herein, “vapor separator” means a vessel or apparatus that can be internal or external to an evaporator system used to separate droplets of process fluid from the evaporated water vapor.
[0043] As used herein, “vapor compressor” means a mechanical compressor used to compress a vapor stream, commonly centrifugal type.
[0044] As used herein, “desuperheater” means an apparatus used to inject water into a stream of superheated steam with the intent of reducing the temperature to the saturated steam temperature.Attorney Docket No. 0036709-000300
[0045] As used herein, “condensate tank” means a vessel used to collect the condensed water vapor from an evaporator system.
[0046] As used herein, “distillate pump” means a pump used to remove the condensed water vapor from an evaporator system.
[0047] Concentrate Pump - A pump used to remove the concentrated process fluid from an evaporator system.
[0048] As used herein “vent condenser” means a heat exchanger used to condense the vent from an evaporator system that uses a liquid solution as the heat sink removing energy from the vent allowing for it to condense.
[0049] Those skilled in the art of evaporator design, design systems that require external heat sources for steady-state operation. In cases where there is incentive to eliminate or substantially reduce the use of external steam those skilled in the art of evaporator design would imagine the use of mechanical vapor compression. As mechanical vapor compression designs utilize the latent heat of vapor from the evaporated water generated to drive evaporation. The energy provided to the mechanical vapor compressor is only what is required to increase the pressure of the vapor to a level such that it can be condensed with the energy being transferred to the process fluid being concentrated.
[0050] In applications that are incentivized to further reduce external make-up steam, heat recovery heat exchangers are implemented to improve the heat balance and thereby reduce the amount of make-up heat required. In some cases, there is a limit to the heat balance improvement that is possible with heat recovery heat exchangers. Embodiments reported herein may provide a novel approach to evaporator design that modifies the evaporator pressure to satisfy the energy balance and thereby provide certainty that external make-up steam will not be required.
[0051] Embodiments typically begin by providing an aqueous stream. This stream could include combinations of inorganic cations and anions and / or may also contain some water-soluble organic compounds. The stream may originate from many sources including an industrial process, a natural resource, or some other source.Attorney Docket No. 0036709-000300
[0052] The water stream is then fed to an evaporator with the intent of either extracting a pure water stream as distilled water or to concentrate the stream for use in another process. Embodiments of this invention consist of any evaporator that is driven by a mechanical vapor compressor. Many types of evaporators may be useful, including but not limited to vertical tube falling film, rising film, flash, horizontal tube falling film, forced circulation, and draft tube evaporator. A multiple-effect evaporator system may also be used.
[0053] Inside the evaporator a fraction of the water stream will be separated as water vapor through evaporation.
[0054] The evaporated vapor flows to the suction of the compressor. The compressor does work on the water vapor increasing the saturation pressure of the water vapor so that the temperature differential between the process fluid and the water vapor is sufficient for heat transfer. The differential temperature required is a function of the surface area available and the heat transfer coefficient of the evaporator for a given evaporation duty. The compressed vapor flows to the heating side of the evaporator where it condenses. As it condenses, the compressed water vapor transfers the energy associated as latent heat of vapor back to the water solution on the process side. The condensate can now be removed from the evaporator. The now concentrated water solution can be removed from the evaporator.
[0055] The operating pressure inside of the evaporator vessel is controlled via the mechanical vapor compressor. The pressure inside the evaporator is controlled to a level that results in the boiling temperature of the solution to be approximately equal to that of the feed such that the energy balance of the system will be positive. This eliminates the need for external make up steam or feed preheaters. See Table 5 below for a representative heat balance and FIG. 7 to see the block flow diagram associated with the example. This method allows for operation of systems that would have otherwise require eternal makeup steam due to limitations with conventional designs.Table 5: Heat Balance of a Heat Neutral EvaporatorHeat Balance of a Heat Neutral EvaporatorFlow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) (°F) (mmbtu / hr) Influent HeatFeed 1,800,000 20.00% 0 165 197,533,727 MVC Power - — 3,827 — 9,740,151 Circulation Pump Power - — 220 — 560,857Attorney Docket No. 0036709-000300 Total -- — — — 207,834,735 Effluent HeatConcentrated Solution 1,607,696 22.39% — 165 171,958,344 Vent Vapors 6,863 — — 173.44 7,793,131 Condensate (Recovered Water) 185,441 0.00% — 173.44 26,228,844 Heat Loss - — — — 1,854,415 Total -- — — — 207.834.735 Heat Balance - — — — 0Makeup Steam Required 0.0 — — — 0
[0056] A vacuum system can also be implemented to assist the MVC in regulating the operating pressure of the evaporator. The vacuum system most commonly consists of a vacuum pump. The vacuum pump may be but is not limited to a liquid ring type vacuum pump. This is generally highly useful for very deep vacuum operation associated with low feed temperatures.
[0057] Operating evaporators under vacuum, particularly deep vacuum, requires significant design changes relative to an evaporator designed at atmospheric pressure for the same service. Due to the large increase in the specific volume of vapor that forms at low pressures, evaporators operating under vacuum require larger vapor paths to accommodate the relatively larger volumetric flow rate of vapor that is generated. The dynamic pressure of a fluid is commonly used to describe its kinetic energy and this value must be limited to avoid significant pressure losses that would diminish the performance of the evaporator. Dynamic pressure can be described by the following equation:(Fluid Density) * (Fluid Velocity)2 / 2 = Dynamic Pressure (3)
[0058] As seen in the calculations below the dynamic pressure of the vapor stream down the tubes of a vertical tube falling film evaporator increases by nearly lOx when operating at a vacuum pressure (1.34 psia in this example) compared to atmospheric pressure of 14.69 psia.Steam Specific Volume at 14.69 psia = 26.8 ft3 / lbSteam Specific Volume at 1.34 psia = 252.4 ft3 / lbMass of Evaporation = 271,429 Ib / hrNumber of Tubes = 2,500Attorney Docket No. 0036709-000300 (Mass of Evaporation) / (# of Tubes) = Vapor flow per tube (4)271,479 Ib / h / 2500 = 108.6 Ib / hr of vapor per tubel / (Fluid Specific Volume) = Fluid Density (5)1 / 26.8 ft3 / lb = 0.037 lb / ft3Atmospheric Case Fluid Density1 / 252.4 ft3 / lb = 0.00396 lb / ft3Vacuum Case Fluid DensityPI*((diameter) / 2)A2 = Cross Sectional Area of Tube (6)PI*((0.167ft) / 2)A2 = 0.022 ft2of Cross Sectional Area per Tube(Mass Flow)* (Specific Volume) / (Cross Sectional Area) = Fluid Velocity (7)108.61b / hr * 26.8ft3 / lb / 0.022ft2* 1 hr / 3600 sec = 36.75 ft / s Atmospheric Case Fluid Velocity108.61b / hr * 252.4ft3 / lb / 0.022ft2* 1 hr / 3600 sec = 346.1 ft / s Vacuum Case Fluid Velocity0.037 lb / ft3* (36.75 ft / s)A2 / 2 = 25 lb / ft*s2Atmospheric Case Dynamic Pressure0.00396 lb / ft3* (346.1 ft / s)A2 / 2 = 237 lb / ft*s2Vacuum Case Dynamic Pressure
[0059] This has significant implications for the evaporator design. To keep the dynamic pressure in the tubes to a reasonable level for vacuum operation the diameter of the tubes would need to be increased from 2in to 3in to manage the higher volumetric flow rate. In our example, the number of tubes would need to be increased as well from 2,500 to 3,500, resulting in a significantly larger vessel diameter, 244 inches versus 140 inches. (Note that the tube length in our example was reduced to maintain the same heat transfer surface area.) This increase in equipment size requirements for an evaporator operating under vacuum versus atmospheric conditions for the same service would be expected to those skilled in the field of evaporator design. This would lead those designers to not prefer such designs because of the larger equipment sizes and larger resulting capital costs. The most notable impact is the reduction in tube length and resulting increase in the number of tubes as increasing the number of tubes significantly increases the amount of labor involved in fabrication. The increase in capital cost can be inferred from the larger sizing requirements described in Table 6.Attorney Docket No. 0036709-000300 Table 6: Equipment Design Comparison Between Conventional MVC Evaporator without Heat Recovery and The Invented MethodDescription Units Atmospheric Vessel Vacuum Vessel Design Design and and Operation Under Operation Vacuum Operating ConditionsFeed Rate Ib / h 950,000 950,000 Feed Temperature F 122 122 Evaporation Rate Ib / h 271,429 271,429 Concentrate Rate Ib / h 678,572 678,572 Concentrate Temperature F 231 131 Operating Pressure psia 14.69 1.34Design InformationHeat Transfer Surface Area ft265,450 65,450# of Tubes # 2,500 3,500 Tube Outer Diameter (OD) in 2 3Tube Length ft 50 24 Dynamic Pressure of Vapor in Tubes lb / (ft*s2) 25 48 Evaporator Shell Diameter in 140 244 Vapor Ducting Diameter before MVC in 62 110 Dynamic Pressure in Vapor Duct before MVC lb / (ft*s2) 173 165 Vapor Ducting Diameter After MVC in 48 78Dynamic Pressure in Vapor Duct After MVC lb / (ft*s2) 275 277
[0060] It is clear from the example above that an evaporator designed at atmospheric pressure for the same evaporation duty will have a lower capital and installation cost. However, a result of the invented method is the ability to eliminate external steam consumption which unexpectedly results in operating cost advantages as demonstrated in table 7 and 8.Table 7: Operating Cost Comparison Between Conventional MVC without Heat Recovery (Supplied with Steam Generated From Natural Gas Boiler) and The Invented Method.Description Units Conventional MVC Invented Method EvaporatorMakeup Steam Requirement Ib / h 51.941 0 Operating Power Consumption kW 5,291 7.236 Operating Power Cost ($0.07 / kW) $ / hr 370 507 Makeup Steam Cost ($10 / ton of Steam with Natural Gas) $ / hr 236 0Total Operating Cost $ / hr 606 507Attorney Docket No. 0036709-000300 Table 8: Operating Cost Comparison Between Conventional MVC without Heat Recovery (Supplied With Steam Generated From Electric Boiler) and the Invented Method.Description Units Conventional MVC Invented Method EvaporatorMakeup Steam Requirement Ib / h 51,941 0 Operating Power Consumption kW 5,291 7,236 Operating Power Consumption of Electric Boiler kW 13,411 0 Operating Power Cost ($0.07 / kW) $ / hr 370 507 Makeup Steam Cost (Based on Electric Boiler) $ / hr 939 0Total Operating Cost $ / hr 1,409 507EXAMPLE
[0061] This example is a prophetic example. A water stream is extracted from a geothermal reservoir at a temperature of 194 °F (90 °C). The water stream contains inorganic and organic species as described in Table 9. The reservoir is located at an elevation of 8,000 ft with an atmospheric pressure of 10.92 psia. The water stream is processed at a rate of 400,000 Ib / hr in a conventional seeded evaporator as well as one employing the invented method.Table 9: Geothermal Brine ChemistryDescription Units Inlet Value Outlet Value Temperature °F 194 194 H2O wt% 85.00 80.00 NaCl wt% 9.01 12.01 KC1 wt% 0.85 0.85 MgC12 wt% 0.75 1 CaC12 wt% 4.22 5.63 CaSO4 wt% 0.21 0.28PFAs ppm78 104
[0062] The water stream consisting of 85% water, 9.01% sodium chloride, 0.85% potassium chloride, 0.75% magnesium chloride, 4.22% calcium chloride, 0.21% calcium sulfate, and 78 ppm polyfluoroalkyl organic compounds is pumped at 400,000 Ib / hr and 75 psig to a seeded evaporator system utilizing the invented method. The feed is pumped to the top head of a vertical tube falling film at pressure sufficient to overcome the pressure losses through the line and the elevationAttorney Docket No. 0036709-000300 differential between the feed tank level and the top head of the evaporator. In this example 75 psig is sufficient to achieve this.
[0063] The water stream then mixes with the circulating fluid within the vertical tube evaporator. The blended solution is then distributed to the heat transfer tubes where it falls as a thin film down the length of the tubes. Inside the tubes, heat is transferred from the water vapor condensing at 11.27 psia in the shell side of the evaporator, to the solution on the tube side at 7.06 psia. This heat transfer induces evaporation of the solution down the length of the heat transfer tubes at a rate of 100,000 Ib / hr. Both the liquid and vapor phase flow down the tubes.
[0064] Disengagement of the two phases occurs in the evaporator sump where the liquid phase falls to the liquid level and mixes with the circulating fluid. The vapor phase disengages the liquid phase as it falls to bulk circulating fluid and the vapor and is pulled up through the mist eliminator mesh pads, mounted in the head space of the evaporator sump. The mesh pads act as a coalescer combining any small water droplets suspended in the vapor phase into larger ones that fall back into the evaporator sump.
[0065] Once through the mist eliminator the evaporated water vapor travels through the vapor ducting and ultimately to the suction of the mechanical vapor compressor. Due to pressure drop through the mist eliminator and the vapor ducting the vapor stream is now at a pressure of 6.90 psia. The vapor compressor, consisting of two (2) centrifugal compressors in series, compresses the vapor stream from 6.90 psia to 11.27 psia. During this compression the vapor stream is superheated, so 3,883 Ib / hr of water is sprayed into the vapor stream leading to its evaporation and saturation of the vapor stream. The now saturated vapor stream flows into the shell side of the evaporator where it condenses on the heat transfer tubes and transfers its energy into the process.
[0066] To ensure non-condensable gases are not accumulating on the shell side of the evaporator, a small vent is taken from the evaporator shell at a rate of 4,000 Ib / hr where the vapor will go from 11.27 psia in the evaporator shell to atmosphere at a pressure of 10.92 psia. The condensed water vapor drains via gravity to the evaporator distillate tank where it is then pumped at a rate of 99,883 Ib / hr and 75 psig. 3,883 Ib / hr of the distillate is directed back the mechanical vapor compressors for de-superheating and the remaining 96,000 Ib / hr is pumped out of the system.The water stream has now been reduced from 400,000 Ib / hr to 300,000 Ib / hr and is continuously removed from the system, via a concentrate pump. This pump discharges at 150 psig, suitable for pumping the concentrated solution through the candle filter for seed recovery and ultimately to the battery limits of the system. The solids separated from the candle filter are recycled back to theAttorney Docket No. 0036709-000300 process to maintain the seedbed in the seeded evaporator. To maintain the level of solids in the seedbed a small bypass is made around the candle filter to purge the required solids from the system. A heat balance of the process described above can be found in Table 10.Table 10: Heat Balance of The Invented Method Processing a Geothermal BrineHeat Balance of The Invented Method Processing a Geothermal BrineFlow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) (T) (mmbtu / hr) Influent HeatFeed 400.000 15.00% 0 194 57,024,000 MVC Power — — 1.838 — 4,677,631 Circulation Pump Power - — 133 — 339,365 Total — — — — 62,040,995 Effluent HeatConcentrated Solution 300,000 20.00% — 192.62 40,476,616 Vent Vapors 4,000 — — 198.90 4.581.836 Condensate (Recovered Water) 96.000 0.00% — 198.90 16,022,543 Heat Loss - — — — 960,000 Total — — — — 62,040,995 Heat Balance - — — — 0Makeup Steam Required 0.0 — — — 0
[0067] Now the water stream consisting of 85% water, 9.01% sodium chloride, 0.85% potassium chloride, 0.75% magnesium chloride, 4.22% calcium chloride, 0.21% calcium sulfate, and 78 ppm polyfluoroalkyl organic compounds is pumped at 400,000 Ib / hr and 95 psig to a conventional evaporator system utilizing feed preheaters. The feed is first split and fed through the feed preheaters that exchange heat between the feed and the effluent distillate and concentrate streams. The distillate enters the preheater at 234.15 °F and leaves at 204 °F. The concentrate enters the preheater at 228.4 °F and leaves at 204 °F. The feed after being preheated to 220.03 °F is sent to the top head of a vertical tube falling film at pressure sufficient to overcome the pressure losses through the line and the elevation differential between the feed tank level and the top head of the evaporator. In this example 95 psig at the feed pump discharge is sufficient to achieve this.
[0068] The water stream then mixes with the circulating fluid within the vertical tube evaporator. The blended solution is then distributed to the heat transfer tubes where it falls as a thin film down the length of the tubes. Inside the tubes, heat is transferred from the water vapor condensing at 22.44 psia in the shell side of the evaporator, to the solution on the tube side at 14.99Attorney Docket No. 0036709-000300 psia. This heat transfer induces evaporation of the solution down the length of the heat transfer tubes at a rate of 100,000 Ib / hr. Both the liquid and vapor phase flow down the length tubes.
[0069] Disengagement of the two phases occurs in the evaporator sump where the liquid phase falls to the liquid level and mixes with the circulating fluid. The vapor phase disengages the liquid phase as it falls to bulk circulating fluid and the vapor is pulled up through the mist eliminator mesh pads mounted in the head space of the evaporator sump. The mesh pads act as a coalescer combining any small water droplets suspended in the vapor phase into larger ones that fall back into the evaporator sump.
[0070] Once through the mist eliminator the evaporated water vapor travels through the vapor ducting and ultimately to the suction of the mechanical vapor compressor. Due to pressure drop through the mist eliminator and the vapor ducting the vapor stream is now at a pressure of 14.70 psia. The vapor compressor, consisting of two (2) centrifugal compressors in series, compresses the vapor stream from 14.70 psia to 22.44 psia. During this compression the vapor stream is superheated, so 3,581 Ib / hr of water is sprayed into the vapor stream leading to its evaporation and saturation of the vapor stream. The now saturated vapor stream flows into the shell side of the evaporator where it condenses on the heat transfer tubes and transfers its energy into the process. Due to the heat imbalance of this system 3,332 Ib / hr of steam at 22.44 psia is also added to the shell side of the evaporator to makeup the required energy deficit and drive evaporation.
[0071] To ensure non-condensable gases are not accumulating on the shell side of the evaporator, a small vent is taken from the evaporator shell at a rate of 4,000 Ib / hr where the vapor will go from 22.44 psia in the evaporator shell to atmosphere at a pressure of 10.92 psia. The condensed water vapor drains via gravity to the evaporator distillate tank where it is then pumped at a rate of 102,913 Ib / hr and 95 psig. 3,581 Ib / hr of the distillate is directed back the mechanical vapor compressors for de-superheating. 3,332 can be directed back to the boilers for makeup water and the remaining 96,000 Ib / hr is pumped through the feed preheater and out of the system. It is important to note that with this level of calcium sulfate in the feed this feed preheater is scaling with calcium sulfate and needs to be cleaned frequently to maintain performance.
[0072] The water stream has now been reduced from 400,000 Ib / hr to 300,000 Ib / hr and is continuously removed from the system, via a concentrate pump. This pump discharges at 150 psig, suitable for pumping the concentrated solution through the candle fdter for seed recovery. The solids separated from the candle fdter are recycled back to the process to maintain the seedbed in the seeded evaporator. To maintain the level of solids in the seedbed a small bypass is made around theAttorney Docket No. 0036709-000300 candle filter to purge the required solids from the system. The concentrated water slurry from the evaporator is then directed to the feed preheater where it is cooled down with feed before leaving the system. It is important to note that the concentrate feed preheater is of a shell and tube design to accommodate the solids in the stream and mitigate plugging. A heat balance of the process described above can be found in Table 11.Table 11 : Heat Balance of a Conventional Evaporator Processing a Geothermal BrineHeat Balance of a Conventional Evaporator Processing a Geothermal BrineFlow Concentration Power Temperature Heat Total (Ib / hr) (wt%) (HP) _ ( F) _ (mmbtu / hr) Influent HeatFeed 400,000 15.00% 0 194 57,024,000 MVC Power - — 1,661 — 4,226,186 Circulation Pump Power -- — 133 — 339,365 Total - — — — 61,589,551 Effluent HeatConcentrated Solution 300,000 20.00% — 204 43,344,001 Vent Vapors 4,000 — — 234.15 4,633,844 Condensate (Recovered Water) 96,000 0.00% — 204 16,511,999 Heat Loss -- — — — 960,000 Total - — — — 65,449,843 Heat Balance -- — — — -3,860,293Makeup Steam Required 3,332 — — 234.15 3,860,293
[0073] When comparing the above example of recovering water from a geothermal between the invented method and a conventional one, there are several key advantages to highlight. First, there are no preheaters required in the invented method. Thus, there will be no downtime associated with cleaning or maintaining these preheaters as will be frequently required when operating on a brine of this quality. Second, there is no need for makeup steam to the evaporator operating with the invented art. As seen in earlier examples this can lead to a large savings in operating costs depending on the steam cost at site.
[0074] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternative embodiments may include some or all the features of the various embodiments disclosed herein. For instance, it is contemplated that a particular feature described, either individually or as part of an embodiment, can be combined with other individually describedAttorney Docket No. 0036709-000300 features, or parts of other embodiments. The elements and acts of the various embodiments described herein can therefore be combined to provide further embodiments.
Claims
Attorney Docket No. 0036709-000300CLAIMSWe claim:
1. A method for water recovery, comprising:a. providing a water stream as feed to an evaporator, wherein the water stream comprises at least one of soluble inorganic compounds and soluble organic compounds;b. feeding the water stream to an evaporator having an evaporator pressure and comprising evaporator heat transfer surface;c. drawing off a fraction of the water stream as water vapor;d. compressing the water vapor and directing the compressed water vapor to the evaporator heat transfer surface as the sole energy source for evaporation;e. controlling the evaporator pressure such that the compressed water vapor condenses on the evaporator heat transfer surface and maintains a positive heat balance;f. removing the condensed water vapor from the evaporator as liquid water;g. removing an effluent water stream from the evaporator at a higher concentration of at least one of soluble inorganic compounds soluble organic compounds relative to the feed stream.
2. The method of claim 1, wherein the concentration factor of the evaporator is less than 2.0.
3. The method of claim 1, where the concentration factor is less than 5.0.
4. The method of claim 1, wherein the evaporator type is a vertical tube falling film evaporator.
5. The method of claim 1, wherein the evaporator has a top head and a bottom head, and wherein the water stream is fed to the top head of the evaporator, travels down the tubes and exits from the bottom head, such that the water stream moves through the evaporator one time without recirculation.
6. The method of claim 1, wherein the evaporator has a top head and a bottom head, and wherein the brine is partially circulated from the evaporator bottom head back to the top head of the evaporator.Attorney Docket No. 0036709-000300 7. The method of claim 1, wherein recirculating brine through the evaporator acts as a heat sink for condensing the water vapor in a shell side vent of the evaporator.
8. The method of claim 1, wherein recirculating brine acts as a heat sink for condensing water vapor prior to a vacuum system.
9. The method of claim 1, further comprising applying heat recovery heat exchangers to the effluent streams allowing for a higher operating pressure to achieve heat neutrality of the system.
10. The method of claim 1, in which the evaporator is in a horizontal tube falling fdm evaporator.
11. The Method of claim 1, wherein the evaporator system is multiple effect.
12. The method of claim 1, wherein the evaporator is a forced circulation type evaporator.
13. The method of claim 1, wherein the evaporator type is a flash evaporator.
14. The method of claim 1, wherein the evaporator type is a rising film evaporator.
15. The method of claim 1, wherein the evaporator type is a seeded evaporator.
16. The method of claim 1, where the heat transfer surface comprises one or more tubes.